This invention relates generally to electronic identification systems which provide the means for the cooperative identification of objects by means of tags attached to or imbedded in the objects. More specifically, the invention relates to identification tags which can cooperate with a variety of types of tag readers.
A key element of an electronic identification system is a means for communication between reader and tag and, since a tag usually has no independent source of power, a means of transferring power from reader to tag in a sufficient amount to permit the tag to perform its functions. These two functions can both be accomplished with electromagnetically-coupled readers and tags, the predominant technology presently in use. The reader establishes an alternating magnetic field in the vicinity of a tag and the tag extracts both information and power from the magnetic field. The efficient extraction of information and power from an alternating magnetic field mandates the use of a resonant circuit of some sort by the tag.
The first close-coupled electronic identification systems, i.e. reader and tag in close proximity when communicating, consisted of readers which transmitted unmodulated carriers and tags which responded with signals that carried data. System capabilities have been extended in recent years with readers that use modulated carriers to transmit data to tags.
The carrier frequencies used by electronic identification systems of the close-coupled variety have ranged from 100 kHz to 2 GHz in the past. Recent efforts at standardization point to a frequency in the 110 to 135 kHz range as being appropriate for worldwide use.
The most significant difference in present-day close-coupled systems is whether the reader is or is not transmitting the carrier when the tag responds with data. Systems in which the reader transmits during the tag response are called "full-duplex" (FDX). Systems in which the reader is silent during the tag response are called "half duplex" (HDX). In an HDX system the reader transmission periods are interlaced with the tag response periods so as to minimize the energy storage requirements in the tag.
A tag transmits data to a reader by modulating a carrier. The frequency of the tag's carrier can be the same as or different from the frequency of the reader's carrier. When the frequencies of the reader's carrier and the tag's carrier are the same, it may seem in some tag designs that the tag is not using a carrier. Instead, the tag is simply modulating the reader's carrier by absorbing more or less energy as a function of time from the alternating magnetic field established by the reader. A better understanding of the communication principles can be had, however, if the details of tag design are ignored in favor of the more general view that the tag creates a modulated carrier with a frequency the same as or different from the frequency of the reader's carrier. The tag's carrier produces a separate alternating magnetic field which is superimposed on the alternating magnetic field established by the reader.
There are a variety of ways in which the reader and tag can modulate their respective carriers with data. One can start with amplitude shift keying (ASK), phase shift keying (PSK), and frequency shift keying (FSK), the names of which indicate the carrier parameter that is modulated. These modulation types are typically used in binary versions wherein the parameter can take on either one of two values. It may become desirable in the future to use n-level forms of these modulation types in order to realize certain communication efficiencies.
The next level of modulation complexity is to combine these basic types of modulation in a variety of ways as, for example, PSK/FSK wherein both the phase and the frequency of a carrier carries data.
A different way of combining modulation types is to piggy-back one modulation type on another as, for example, when a subcarrier is frequency shift keyed in accordance with the bits in a message, and then the carrier is amplitude modulated by the FSKed subcarrier.
The communications between reader and tag are in the form of messages consisting of a finite number of bits. Each message bit is usually translated into one or more transmit bits prior to modulating a carrier. The typical translations include (besides the identity translation where the message bits are also the transmit bits):
Manchester--0 translates into 01; 1 translates into 10; PA0 Miller--T(N,1)=T(N-1,2) EX.OR [Mbar(N-1) AND Mbar(N)]
T(N,2)=T(N,1) EX.OR M(N)
where M(N) is the N'th message bit, Mbar(N) is M(N) inverted, and T(N,1), T(N,2) are the first and second transmit bits associated with the N'th message bit.
Electronic identification systems which utilize implantable or attachable tags have proliferated over the past decade to the point where users are seriously inconvenienced by the incompatible equipments produced by vendors who participate in this market. In general, tags supplied by one vendor cannot be read by the readers supplied by another vendor which means that users necessarily find themselves locked into the systems of one manufacturer. For large-scale applications of electronic identification to occur, some means for assuring equipment compatibility is essential.
There are a number of avenues that can be followed in achieving equipment compatibility. The typical approach to achieving interoperability of equipments is the establishment of standards for this purpose. The establishment of standards has the disadvantage of tending to freeze technology and hinder the development of more advanced systems.
Another approach is to make available "universal" tag readers which can read the tags that are presently being used, and which can be economically upgraded to reading tags that are developed in the future.
A third approach is to make available "universal" tags which can be read by any reader that is presently being used, and which can be upgraded for use with readers of different designs that appear in the future.